Process for preparing disilane from monosilane by electric discharge and cryogenic trapping and new reactor for carrying it out

ABSTRACT

The invention relates to a process for producing disilane from monosilane, according to which gaseous monosilane is passed into a reaction zone where it is subject to an electric discharge generated by a high-frequency current.

This is a division of parent application Ser. No. 08/206,847 filed Mar.7, 1994 now U.S. Pat. No. 5,478,453.

SUMMARY OF THE INVENTION

The invention relates to a process for producing disilane frommonosilane, according to which gaseous monosilane is passed into areaction zone where it is subjected to an electric discharge generatedby a high-frequency current, characterised in that

a) the monosilane is used in the form of a mixture with at least oneinert gas selected from the group formed by helium and argon,

b) the pressure of the gaseous mixture in the reaction zone is between0.1 and 3 bar, and

c) the gaseous mixture is brought into contact, in the reaction zoneunder electric discharge, with a wall cooled to a temperature which issufficiently low for the saturation vapour pressure of the disilane tobe negligible, but not low enough for the monosilane to be condensableat the working partial pressure, and to a reactor for carrying it out.

Used in particular by the electronics industry.

The invention relates to a process for preparing disilane frommonosilane by electric discharge and cryogenic trapping.

The use of disilane in the microelectronics industry to depositsemi-conducting amorphous silicon has very considerable advantages, i.e.increasing the deposition rates and reducing the deposition temperature.These advantages may be considerable, but they come up against theexcessively high cost of disilane (10 times higher than that ofmonosilane).

Disilane is prepared at present with low yields by the chemical reactionof metal silicides with inorganic acids, or of hexachlorosilane withaluminium and lithium hydride.

Commercially available disilane is not only expensive, but is also oftencontaminated by impurities, such as chlorosilanes, siloxanes andhydrocarbon compounds.

A process for producing disilane from monosilane using a glow dischargeand carried out at a very low pressure is also known from U.S. Pat. No.568,437. However, the disilane yield obtained is only average(approximately 40% at best) and the fact that it is necessary to work invacuo makes it difficult for a process of this kind to be used on anindustrial scale.

It would therefore by very useful to have a process for producingdisilane which gives good disilane yields free of troublesome impuritiesand is economical to carry out.

The aim of the invention is, inter alia, to provide a process of thiskind.

The invention relates to a process for producing disilane frommonosilane, according to which gaseous monosilane is passed into areaction zone where it is subjected to an electric discharge generatedby a high-frequency current, characterised in that

a) the monosilane is used in the form of a mixture with at least oneinert gas selected from the group formed by helium and argon,

b) the pressure of the gaseous mixture in the reaction zone is between0.1 and 3 bar, and

c) the gaseous mixture is brought into contact, in the reaction zoneunder electric discharge, with a wall cooled to a temperature which issufficiently low for the saturation vapour pressure of the disilane tobe negligible, but not low enough for the monosilane to be condensableat the working partial pressure.

The process of the invention is based on the joint use of aplasma-forming electric discharge and a cryogenic trap situated in thereaction zone itself.

The electric discharge, which is of the type referred to as a silentdischarge or corona discharge, acts on the monosilane molecules tocreate ions, radicals and excited molecules, and these species reactwith one another initially to form disilane, which can then be convertedinto trisilane, then into polysilanes and finally into hydrogenatedamorphous silicon particles. According to the invention, a cryogenictrap situated in the reaction zone itself is provided in order to trapthe disilane as it forms and before it is itself converted to aconsiderable degree into higher species.

The absolute pressure of the gaseous mixture can be from 0.1 to 3 bar,preferably 1 to 1.3 bar. Below 0.1 bar and above 3 bar, the equipmentrequired becomes too cumbersome. In addition, above 3 bar, theproportion of monosilane in the gaseous mixture is limited to lowlevels, otherwise the voltage required to start the discharge may becometoo high and cause breakdowns.

The composition of the gaseous mixture depends largely on the operatingpressure, as it has been found that the partial pressure of the startingmonosilane must be situated between 0.01 and 0.1 bar, preferably between0.04 and 0.08 bar, a partial pressure of less than 0.01 bar giving a lowyield of the desired disilane, and on the fact that the starting voltagefor the electric discharge is an increasing function of the partialpressure of the silane in the reaction zone, a partial monosilanepressure of more than 0.1 bar requiring a high starting voltage for theelectric discharge which can cause breakdowns which may damage theequipment.

If, therefore, the process is carried out at a pressure close to thelowest possible pressure (0.1 bar), monosilane can constitute the vastmajority of the gaseous mixture. In the preferred case in which theprocess is carried out at atmospheric pressure or at a slightly higherpressure, the gaseous mixture advantageously contains 1 to 10%,preferably 4 to 8% by volume of SiH₄ and 90 to 99%, preferably 92 to 96%by volume of the said inert gas.

It should be noted that, in addition to monosilane and the inert gas,the gaseous mixture can include a small quantity (e.g. less than 10% byvolume) of hydrogen, without this having any adverse effect on theprocess.

The frequency of the electric current would appear to influence thedisilane yield although the reasons for this are not clear. E.g. it hasbeen found that with the installation available and describedhereinbelow, a frequency of 3 kHz greatly improves the disilane yieldcompared to a frequency of 50 kHz. A frequency of 1 to 10 kHz, inparticular 2 to 5 kHz, would therefore appear to be advisable inaccordance with our present knowledge.

The residence time of the gaseous mixture in the reaction zone isadvantageously short, e.g. less than 10 seconds, preferably less than 4seconds and better still less than 2 seconds.

Moreover, the temperature of the cold wall is very important. It must besufficiently cold for the saturation vapour pressure of the disilane tobe negligible, but insufficiently cold for the monosilane to becondensable at the partial working pressure. It has been found that acold wall temperature from -120° C. to -145° C. is usually satisfactoryfor the purposes of the invention.

The conditions stipulated hereinabove are of course only given by way ofexample and are in no way limiting.

Disilane yields (number of moles of disilane formed compared to thetheoretical maximum number of moles) greater than 50% and able to reachvalues as high as 95% and more can be obtained with the process of theinvention.

The invention also relates to a new reactor which can be used, interalia, to carry out the process of the invention.

More precisely, the invention relates to a reactor comprising a reactionchamber, a conduit for continuously supplying this chamber with agaseous mixture, a conduit for continuously discharging the residual gashaving traversed the reaction chamber and means for establishing anelectric discharge through the reaction chamber, comprising a pair ofelectrodes and a high-frequency electric current source, characterisedin that the reaction chamber is delimited at least partially by a wallwhich can be cooled to a very low temperature.

The wall is preferably cooled to a temperature of -120° C. or lower.

According to one particular preferred embodiment, the reaction chamberis delimited by two concentric cylindrical elements, wherein the innercylindrical element is made of a dielectric material and is providedwith a metal coating on its surface opposite the outer cylindricalelement, and the outer cylindrical element is made of a metal and can becooled externally by means of a coil traversed by a coolant.

By virtue of the fact that a metal-coated dielectric material is used toform both one of the walls of the reaction chamber and one of theelectrodes, it is possible to obtain a uniform electric discharge and toprevent the formation of hot spots in the plasma formed.

The outer cylindrical element is advantageously also provided with anelectric heating resistor so that it is possible to regulate thetemperature of the cooled wall to a selected temperature set pointgreater than that normally produced by the coolant by alternatelypassing the coolant into the coil and operating the heating resistor asrequired.

The invention will now be described with reference to the accompanyingdrawings and the examples given hereinafter.

In the drawings, the single FIGURE is a diagrammatic sectional view of areactor according to the invention.

This reactor, designated in general by the reference numeral 1, isformed by a sealed enclosure formed by an outer wall 2 and an inner wall3 of stainless steel at a distance from one another, between which thevacuum has been created for the purposes of thermal insulation. Aconcentric cylindrical element 4 is disposed in the interior of thelower part of the inner wall 3, these defining between them a reactionchamber 5, this lower part moreover being provided on its outer facewith a cooling coil 6 and an electric heating resistor 7 wound around 6.A conduit 8 for supplying liquid nitrogen and a conduit 9 fordischarging the gaseous nitrogen allow a cold current of gaseousnitrogen to be established in the coil. The inner cylindrical element 4,the base 4a of which is closed, is made of a dielectric material, e.g.alumina or borosilicate glass, and is provided on its face opposite thewall 3 with a metal coating 10, e.g. a silver-plated layer, connected bya contact 11 and a conductor 12 to a high-frequency electric source 13.The metal wall 3 for its part is connected to earth.

The reactor also comprises a conduit 14 for supplying the startinggaseous mixture, which traverses the base 4a of the element 4 and opensinto a space 15 formed between the base 4a of the element 4 and the baseof the wall 3, and a conduit 16 for discharging the residual gaseousmixture originating in the part of the enclosure situated above thechamber 5.

The cylindrical element 4 is held at a distance from the wall 3 and fromthe base of the enclosure by spacing elements 17 (forming an integralpart of the element 4) distributed at regular intervals. The gaseousflux can pass between them.

By way of example, the reactor used in the following examples comprisedcylindrical elements having a diameter of approximately 3 cm forming aspace between them forming a reaction chamber, and having a thickness ofapproximately 1 mm. The height of the chamber was varied from 1 to 3 cmby simple exchange of the cylindrical element 4. The thickness of thedielectric material (alumina) forming the element 4 was approximately1.5 mm.

EXAMPLES

Synthesis of disilane was carried out several times with the aid of thereactor just described. The operating conditions and the resultsobtained are summarised in the Table hereinafter.

The disilane formed condensed on to the cooled wall and remained on thiswall in the form of a film and/or fell to the bottom of the reactor. Atthe end of the reaction period, the supplies of gaseous mixture andcoolant were stopped and the reactor was purged with helium in order toremove the residual monosilane, then the disilane was collected in theform of a gaseous mixture with helium and a little trisilane naturallyreleased from the reactor during the heating thereof. The gaseousmixture collected typically had the following composition, inpercentages by volume:

disilane 60%

trisilane 1%

residual monosilane 1%

helium 38%.

As a variant, the reactor could be provided with means for recoveringthe product formed in the liquid state, e.g. by providing a bleed valveat the bottom of the reactor.

The following Table clearly shows the high disilane yields which can beobtained with the process of the invention.

The embodiments described are of course only given by way of examplesand could be amended, inter alia, by substituting equivalent techniqueswithout thereby going beyond the scope of the invention.

In particular, although the reactor of the invention has been describedin association with the synthesis of disilane, it should be noted thatthe usefulness of this reactor is not limited to this specificapplication, but that it could be used for the synthesis of any chemicalspecies which can be produced in an electric discharge, and that it isadvantageous to trap it as it forms and before it is converted intoother species.

It would also be possible to operate two reactors of the type describedin alternation, one of them supplying a disilane application produced ina preceding synthesis operation, while the other is in the process ofsynthesising disilane, so that the user has a continuous source ofdisilane.

                  TABLE                                                           ______________________________________                                                      Flow rate.sup.(2)                                                                       Compo- Pressure.sup.(4)                                                                      Height.sup.(5)                         Ex.  T, °C..sup.(1)                                                                  cm.sup.3 /min                                                                           sition.sup.(3)                                                                       bar     cm                                     ______________________________________                                         1   -135      50       95/5   1.05    3                                       2   -135     100       95/5   1.31    3                                       3   -135     100       95/5   1.25    3                                       4   -135     100       95/5   1.25    3                                       5   -135     100       95/5   1.25    3                                       6   -135     100       95/5   1.25    3                                       7   -135     100       95/5   1.25    3                                       8   -135     100       95/5   1.25    3                                       9   -135     100       95/5   1.25    3                                      10   -135     100       93/7   1.25    3                                      11   -135     100       95/5   1.1     1                                      12   -135     100       95/5   1.1     1.5                                    13   -135     100       95/5   1.1     1.5                                    14   -135     100       95/5   1.1     1.5                                    15   -135     100       95/5   1.1     1.5                                    A(   -150     100       95/5   1.25    3                                      B(   -100     100       95/5   1.25    3                                      (outside the invention)                                                       ______________________________________                                                  Power.sup.(6)                                                                          Frequency.sup.(7)                                                                        Duration.sup.(8)                                                                      Yield.sup.(9)                           Example   W        kHz        min     %                                       ______________________________________                                         1            --       50       30      56.5                                   2            --       50       30      57.3                                   3            12.6     3        30      75.3                                   4            10.5     3        30      76                                     5            8.5      3        30      84                                     6            4.5      3        30      70.7                                   7            4.5      3        35      76.2                                   8            4.5      3        60      70.4                                   9            4.5      3        150     56.7                                  10            3.2      3        30      64.4                                  11            4.2      3        50      55.4                                  12            4        3        50      83.2                                  13            5.2      3        50      88.4                                  14            5.4      3        50      90                                    15            6        3        50      95                                    A   (outside  N.A.     50       30      ˜0                              B   (the      N.A.     50       30      6.17                                      (invention)                                                               ______________________________________                                         Notes:                                                                        .sup.(1) temperature of the cold wall                                         .sup.(2) flow rate of the supply of gaseous mixture                           .sup.(3) composition of the helium/monosilane gaseous mixture in % by         volume                                                                        .sup.(4) pressure prevailing in the reaction chamber                          .sup.(5) height of the inner cylindrical element                              .sup.(6) electrical power consumed, as determined by calorific                measurements. The intensity of the electric current varied from 0.5 to 3      mA as the case may be                                                         .sup.(7) frequency of the electric current maintaining the electric           discharge                                                                     .sup.(8) duration of the example                                              .sup.(9) disilane yield compared to the theoretical quantity             

We claim:
 1. Reactor comprising a reaction chamber (5), a conduit (14)for continuously supplying this chamber with a gaseous mixture, aconduit (16) for continuously discharging the residual gas havingtraversed the reaction chamber and means for establishing an electricdischarge through the reaction chamber, comprising a pair of electrodes(3, 10) and a high-frequency electric current source (13), characterisedin that the reaction chamber is delimited at least partially by a wall(3) connected to a cooling means that cools wall (3) to a temperature of-120° C. or lower.
 2. Reactor according to claim 1, characterised inthat the reaction chamber is defined by two concentric cylindricalelements (3, 4), wherein the inner cylindrical element (4) is made of adielectric material and is provided with a metal coating (10) on itssurface opposite the outer cylindrical element (3), and the outercylindrical element (3) is made of a metal and can be cooled externallyby means of a coolant.
 3. Reactor according to claim 1, characterised inthat the outer cylindrical element (3) is also provided with an electricheating resistor (7) so that it is possible to regulate the temperatureof the cooled wall to a desired temperature set point.